Why is this important?

Microsoft Internet Explorer was the most popular browser that
implemented P3P. With Windows 10, P3P's support was removed
entirely from Internet Explorer 11 and has minimal servicing
for other versions of Windows. Other popular
browsers never implemented or removed this feature before Microsoft
did.

On top of the lack of support, if the header is sent and it’s not
kept in sync with normal human-readable privacy policies, it may be
a cause of legal confusion, which might present legal risks. Please
check with a local lawyer to see if that’s the case in your country.

Additionally, studies have detected that about 33% of
sites using P3P don’t have a valid configuration. In some cases,
the value was used to circumvent Internet Explorer cookie blocking
(and thus rendering P3P ineffective). Others had typos and errors
in the tokens.

Because of all the above reasons it’s recommended to not use P3P
anymore.

One thing to keep in mind if you need to support old versions of
Internet Explorer is that:

By default, Internet Explorer will reject cookies coming from
3rd-party contexts. A 3rd-party context is one where the domain
on the content is different than the domain of the page that pulls
in that content. Possible third-party contexts include pretty much
any element that accepts a URL: <script>, <img>, <link>,
<frame>, <iframe>, <audio>, <video>, et cetera. It also
includes cross-domain XMLHttpRequest which attempt to send
cookies when the withCredentials flag is set.